1. Field of the Invention
This invention relates to temperature monitoring apparatus using microwave radiometry. It relates more particularly to apparatus for non-invasively and reliably detecting and monitoring the temperature of target brain tissue especially in neonatal patients.
2. Background Information
Hypoxic-ischemia or asphyxia is a serious problem in newborns. Lack of an adequate supply of oxygenated blood to the brain may result in serious brain injury called hypoxic-ischemic encephalopathy (HIE). It is estimated that HIE occurs in about 3-9 newborns out of 1,000 term infants. About half of these infants with HIE (range of 35-50%) will die or suffer from severe handicaps such as mental retardation or cerebral palsy.
At the present time, beyond the initial care given to the newborn in the delivery room to provide artificial breathing or medications, there is no standard care to lessen HIE. Multi-centered trials are currently underway in the United States and elsewhere to measure the safety and efficacy of hypothermia in reducing brain injury events following periods of low oxygen to the brain. The objective is to cool the brain by 4-33° with an accuracy of about 0.20° C. Sorely needed, then, is a non-invasive sensor which can reliably detect and monitor temperature of the target brain tissue.
Currently, trials effecting hypothermic cooling of the head or systemic hypothermia utilize rectal temperature as an indirect index of the infant's brain temperature. However, while performing hypothermia, it is necessary not only to control brain temperature but also to control the rate of cooling, as well as the rate to re-warm the brain. Such close control cannot be maintained by measuring temperature at a remote site such as the rectum.
It is also possible to measure deep brain temperature non-invasively using magnetic resonance spectroscopy, but the cost and feasibility are big problems with this approach considering the need for continuous 72-hour thermal monitoring of each newborn in a ferromagnetic-free environment, followed by a 16-24 hour re-warming period, bringing the total time for each hypothermia procedure to about 96 hours.
The monitoring of axillary esophageal and nasopharyngeal temperatures to indicate intracranial temperature has also been considered. However, these areas like the rectal area are slow in indicating thermal trends in the brain.
There are also available devices based on infrared (IR) technology which may be placed in the ear. The tympanic membrane temperature is considered a more reliable indicator of brain temperature than oral or rectal readings because it is closer to the brain. However, these so-called tympanic devices, although relatively inexpensive, are still not considered sufficiently accurate for this application. This is because such devices show an inability to deliver an accurate temperature reading for various reasons such as wax and moisture in the ear which are absorptive at IR frequencies. Also, such devices actually measure radiation from the ear canal wall and the environment within the ear canal resulting in a lower temperature reading than would be obtained from the tympanic membrane itself. Still further, the available IR devices are physically large and therefore not suited for the ear of a neonatal patient who, as noted above, may require continuous monitoring for many hours. Finally, commercially available IR devices do not read below 34° C., while the preferred temperature range for intracranial cooling is 32.8-33.0° C.
In sum, the prior apparatus are disadvantaged in that they do not measure the brain parenchymal tissue directly but rather rely on indirect temperature measurements on other parts of the body and they do not also measure the surface temperature at the cranium thereby allowing the patient's skin to be over-cooled during the hypothermic treatment.
Accordingly, it would be desirable to provide apparatus for reliably detecting and monitoring brain temperature at depth as well as near surface temperature directly but non-invasively during long-term hypothermic treatment to reduce the incidence of brain injury particularly in neonates.